The present invention is related to electron guns. More specifically, the present invention is related to an electron gun that uses an electrostatic field to radially focus and axially accelerate a DC electron beam.
The development of reliable, non-contaminating and long-life (robust) high-current electron beam sources for injection into klystrons and related devices has been a challenging problem for many years. High-current beams are widely used in injector systems for electron accelerators, both for industrial linear accelerators (linacs) and high-energy accelerators. High-current electron beams are also used for microwave generation (in klystrons and related devices), for research on advanced methods of particle acceleration, and for injectors used for free-electron laser (FEL) drivers. During the last few years considerable effort has been applied to the development of high power linac injectors [J. L. Adamski et al., IEEE Trans. Nucl. Sci. NS-32, 3397 (1985); T. F. Godlove, et al, Part. Accel. 34, 169 (1990)] and particularly to laser-initiated photocathode injectors [J. S. Fraser and R. L. Sheffield, IEEE J. Quantum Elec. QE-23, 1489 (1987); P. Schoessow, E. Chojnacki, W. Gai, C. Ho, R. Konecny, S. Mtingwa, J. Norem, M. Rosing, and J. Simpson, Proc. of the 2nd Euro. Part. Accel. Conf. p. 606 (1990)]. The best of the laser injectors have relatively high beam quality, but their reliability depends on the choice of photocathode material, with the more reliable materials requiring intense laser illumination.
The high-density electron gun invention to be described here is called a Robust Pierce Gun (RPG). [See xe2x80x9cTheory and Design of Electron Beamsxe2x80x9d, J. R. Pierce, D. Van Nostrand Company, Inc. (1954)]. The RPG avoids the difficulties associated with plasma cathodes, thermionic emitters, and field emission cathodes. Plasma cathodes cannot be operated at high repetition rate, nor can they sustain very long pulses without voltage collapse. Thermionic emitters are only good for low current densities ( less than 20 Amps/cm2), and are easily contaminated. Field emission cathodes require a huge field (xcx9c109 MV/m) for reasonable emission. Laser-initiated photocathodes require an expensive laser system and suffer from reliability issues in high electric fields.
High current-density beam generation methods used to date are rather complex, cumbersome, expensive, and have very definite limits on performance. The RPG described here is promising in large part because of the natural current amplification process inherent in secondary electron emission. This natural amplification process makes possible a simply-designed gun which could provide a cold cathode at high-current densities operating at modest duty factors and relatively high-quality pulsed electron beams suitable for many applications.
The present invention relies upon amplifying, by means of secondary electron emission, a beam of electrons produced by a reliable low-current-density electron emitter. The invention is based on the phenomenon of transmitted secondary electron production from surfaces of negative-electron-affinity (NEA) materials [R. U. Martinelli and D. G. Fisher, Proc. of the IEEE 62, 1339 (1974); H. Bruining, Physics and Applications of Secondary Electron Emission (Pergamon Press, London, 1954), incorporated by reference herein]. A beam of electrons (primary beam) is accelerated in a cathode/anode configuration to impinge on a film electrode (which has a thickness to allow the transmission mode of operation) of an NEA material. Depending on the range of the electrons in the film electrode, secondary electrons are then created preferentially on the backside of the thin film electrode, that is, in the direction of propagation of the primary beam. Current amplification through one stage of a NEA material like diamond could be increased by a factor of 50. To accomplish amplification of the electron current density, one or more stages of secondary emitter films are utilized along with one primary emitter. The primary emitter is a low-current-density robust emitter (e.g., thoriated tungsten). Examples of NEA materials are GaAs, GaP, Si, diamond, and materials used as photoemitters, secondary electron emitters, and cold-cathode emitters.
The first component of the present invention pertains to the electron gun. The electron gun comprises an electrostatic cavity having a first stage with emitting faces and multiple stages with emitting sections. The gun is also comprised of a mechanism for producing an electrostatic force which encompasses the emitting faces and the multiple emitting sections so electrons are directed from the emitting faces toward the emitting sections to contact the emitting sections and generate additional electrons and to further contact other emitting sections to generate additional electrons and so on, then finally to escape the end of the cavity.
The emitting sections preferably provide the cavity with an accelerating force for electrons inside the cavity. The multiple sections preferably include forward emitting surfaces. The forward emitting surfaces can be of an annular shape, or of a circular shape, or of a rhombohedron shape.
The mechanism preferably includes a mechanism for producing an electrostatic electric field that provides the force and which has a radial component that prevents the electrons from straying out of the region between the first stage with emitting faces and the multiple emitting sections. Additionally, the gun includes a mechanism for producing a magnetic field to contain the electrons anywhere from the first stage with emitting faces or any emitting section and to the end of the cavity.
The first component of the present invention pertains to a method for producing a flow of electrons. The method comprises the steps of moving at least a first electron in a first direction at one location. Next there is the step of striking a first area with the first electron. Then there is the step of producing additional electrons at the first area due to the first electron. Next there is the step of moving electrons from the first area to a second area and transmitting electrons through the second area and creating more electrons due to electrons from the first area striking the second area. These newly created electrons from the second area move in the first direction then strike the third area, fourth area, etc. Each area creates even more electrons in a repeating manner by the electrons moving in the first direction to multiple areas. This process is also repeated at different locations.
The mechanism preferably includes a mechanism for accelerating the electrons inside the cavity to allow the electron multiplication to continue.
The electron preferably includes a control grid for interrupting the flow of electrons and thus to create bunching of the electrons.
The present invention pertains to an electron gun. The electron gun comprises an electrostatic cavity having a first stage with electron emitting faces and multiple stages with electron emitting sections. The electron gun also comprises a mechanism for producing an electrostatic force which encompasses the electron emitting faces and the multiple electron emitting sections so electrons from the electron emitting faces and sections are directed from the emitting faces toward the emitting sections to contact the emitting sections and generate additional electrons on the opposite sides of the emitting sections and to further contact other emitting sections.
The present invention pertains to a method for producing electrons. The method comprises the steps of moving at least a first electron in a first direction from a first location. Then, there is the step of striking a first area with the first electron. Next, there is the step of producing additional electrons at the first area due to the first electrons on the opposite side of the first area which was struck by the first electron. Next, there is the step of moving electrons from the first area to a second area. Then, there is the step of transmitting electrons to the second area and creating more electrons due to electrons from the first area striking the second area.